1. Field of the Invention
The present invention relates to an apparatus for processing sewage. More particularly, the present invention is a horizontally-rotating, multi-pass, continuous-feed digester which is suitable for treating sewage in general, but which is particularly useful in accomplishing aerobic digestion processes.
The digestion of sewage often has been carried out in large open tanks, wherein the process has been difficult to control. Large tanks are difficult to aerate uniformly, somewhat difficult to control temperatures, and it is also difficult to separate the input and the output. Consequently, the prior-art processes have generally not been efficient and require long holding times as a result of which the throughput is very low.
In most such sewage treatment plants, residues of heavy sludge, which comprise less than 0.2 percent of water-borne sewage, have been diverted to sedimentation tanks where relatively slow aerobic decomposition continues. The sludge was usually held in such digestion tanks for several weeks and was there finally digested into a dark, viscous fluid within which theoretically little or no further biological action could occur. The sludge was then removed from the tank and deposited on sludge-drying beds, dewatered in large heated drums, pulverized, sold as fertilizers, or incinerated. The heat generated in such processes kills any enteric pathogens.
2. Description of Related Art
Expired U.S. Pat. No. 3,720,320 (Fletcher) discloses a marked departure from the somewhat cumbersome, slow, digestion of sludge as described above. Fletcher describes a multi-pass device which supports an aerobic digestion process for decomposing sewage. The raw sewage was fed into this digester where it was aerated while rotating on a horizontal axis so that biological metabolism by the bacteria occurred in the sludge.
However, Fletcher has not achieved commercial success, possibly because first, air, and/or other gas, hereinafter "air/gas" enters at one end and then exits as exhaust gas at the same end of the digester. That may permit contamination of freshly oxygenated gas if a ring seal in a slip fitting should fail. Secondly, a like risk exists with the effluent in that raw sewage and effluent enter and discharge at the same end of the digester permitting contamination should a ring seal fail in the slip fitting, and thirdly, in applicant's improved digester the flow of sewage is modulated depending on the level or head of the feedstock.
A second serious problem with that model is that the air/gas distribution pipes, baffles, agitator blades and air/gas diffusers within the digester cannot be cleaned without shutting down the entire system.
A third problem to be overcome is met by the inclusion of a regulator valve which modulates the influx of sewage responsive to variable back pressure due to static head in the digester.
My improved invention resolves these problems by: (1) providing easily-removable means for aeration so that nozzles can be inspected and cleaned while digestion continues; (2) diverting sewage and air/gas so that they exit and enter at opposite ends of the digester; and (3) providing a regulator valve which is sensitive to and controls the total volume of sewage entering the digester; and (4) providing a valve that commutates the distribution of air/gas to a plurality of air headers to optimize the amount of air/gas delivered depending upon the static head variation as the digester rotates.
It will be seen that such a digester can process a biomass made up from wastes from food-processing, plant materials and animal waste. The input material will be referred to generically as "feedstock" or "raw sewage", the contents reacting therein as "biomass" and the material towards the end of the process as "effluent".